The invention relates to a de-clutching mechanism for belt-driven transmissions, such as are used in small tractors like self-propelled lawnmowers.
Small tractors, like self-propelled lawnmowers, include a gearbox which carries a gear shift mechanism. The gears are driven by an input shaft which is fixedly connected to a drive pulley, the pulley being driven by a belt that is rotated by a motor.
In order to shift gears, it is necessary to declutch in order to interrupt the power being transmitted to the input shaft. That has traditionally been performed by loosening the belt tension to permit the belt to slip relative to the drive pulley.
Such an arrangement involves a number of shortcomings. The loosening of the belt gives rise to a risk that the belt can become dislodged from the drive pulley. Also, the slippage of the belt relative to the drive pulley produces appreciable wearing of the belt, thereby shortening the belt life. Special belt design techniques have been implemented to reduce the loss of belt life but the result is relatively expensive belts. Also, the mechanisms for loosening/tightening the belt can be relatively complicated and expensive.
In addition, when it is desired to change gears as the tractor is moving downhill, the large gear ratio typical of such gearboxes means that even the slightest force applied to the pulley by the belt will make it difficult to effect a smooth gear shift. Therefore, it would be desirable to ensure that smooth gear shifting can be effected at all times, as well as to increase belt life, reduce belt cost, and minimize the risk of the belt becoming dislodged from the pulley.